Known is a motor unit provided with a rotation detector that detects a rotation of a shaft using magnetism. As such a rotation detector, a rotation detector using magnetism is known.
For example, the rotation detector (magnetic sensor for detecting rotations) illustrated in FIG. 7 in Japanese Patent Application Laid-open No. 2001-194182 (hereinafter, referred to as Document 1) includes a detector element that is a detection coil (11) wound around a wire-like magnetic element (10) in which a large Barkhausen jump can occur, and a drum-like base (20) whose rotational central axis (21) is connected to the target of detection, e.g., a motor shaft. In the drum-like base (20), a plurality of permanent magnets (31 to 36) of alternating polarities are arranged side by side at an equal interval. In this rotation detector, when the drum-like base (20) is rotated, the permanent magnets (31 to 36) sequentially pass a position near the wire-like magnetic element (10), whereby providing an alternating field to the wire-like magnetic element (10). As a result, the direction in which the wire-like magnetic element (10) is magnetized is switched sequentially, causing the detection coil (11) to output pulse signals indicating rotational conditions of the drum-like base (20).
In the rotation detector illustrated in FIG. 7 in Document 1, the wire-like magnetic element (10) extends in a direction parallel with the rotational axis of the drum-like base (20). Therefore, the size of the rotation detector in the direction along the rotational axis is large. Therefore, if such a rotation detector is mounted on a motor unit, the size of the motor unit is increased, disadvantageously.
By contrast, the rotation detector (rotation sensor) illustrated in FIG. 1 in Japanese Patent Application Laid-open No. 2000-161989 (hereinafter, referred to as Document 2) includes a sensor coil (2) formed by winding a coil wire (22) around a core metal (21) made of an amorphous magnetic material having a Barkhausen effect, and permanent magnets (1) that are attached to a rotational plate (200) rotating in a predetermined rotating direction. The axial direction of the core metal (21) in the sensor coil (2) is positioned in parallel with a direction tangential to the rotating direction of the rotational plate (200). Therefore, the rotation detector illustrated in FIG. 1 in Document 2 can reduce the size in the direction of the rotational axis, compared with the rotation detector illustrated in FIG. 7 in Document 1. Such a rotation detector can also be mounted on a motor unit so as to allow the size reduction of a motor unit to be achieved.
However, there are disadvantages such as those to be described below in a rotation detector in which the sensor coil (2) (magnetic field detector) is positioned so that the longitudinal direction of the core metal (21) (magnetic element) is laid in parallel with a direction tangential to the rotating direction of the rotational plate (200) on which the permanent magnets (1) are attached, as illustrated in FIG. 1 in Document 2.
For example, when an N-pole permanent magnet (1) and one end (2a) of the sensor coil (2) are brought near to each other as the rotational plate (200) is rotated, as illustrated in FIG. 2(a) in Document 2, because the magnetic field formed by the permanent magnet (1) passes through the core metal (21) in a direction from the one end (2a) toward the other end (2b) of the sensor coil (2), the core metal (21) is magnetized to one direction. When the N-pole permanent magnet (1) and the other end (2b) of the sensor coil (2) are brought near to each other as the rotational plate (200) is rotated, as illustrated in FIG. 2(b) in Document 2, because the magnetic field formed by the permanent magnet (1) passes through the core metal (21) in a direction from the other end (2b) toward the one end (2a) of the sensor coil (2), the core metal (21) is magnetized to the opposite direction of the one direction. A pulse signal corresponding to the direction in which of the core metal (21) is magnetized is output from the coil wire (22) included in the sensor coil (2).
The rotational conditions of the rotational plate (200) can be detected accurately if the magnetized direction of the core metal (21) changes only when the permanent magnet (1) and the one end (2a) of the sensor coil (2) are brought near to each other and when the permanent magnet (1) and the other end (2b) of the sensor coil (2) are brought near to each other as the rotational plate (200) is rotated. However, the magnetized direction of the core metal (21) could change in cases other than those described above.
To explain more, when a longitudinal-direction mid-portion of the permanent magnet (1) and a longitudinal-direction mid-portion of the sensor coil (2) are brought near to each other as the rotational plate (200) is rotated, the magnetized state of the core metal (21) could become unstable, and cause the magnetized direction of the core metal (21) to change. Such a change in the magnetized direction does not always occur when the longitudinal-direction mid-portion of the permanent magnet (1) and the mid-portion of the sensor coil (2) are brought near to each other, but sometimes occurs and sometimes not, and it is difficult to predict whether such a change in the magnetized direction will occur.
A possible cause of such a change in the magnetized direction of the core metal (21) that is difficult to predict is that the direction of the magnetic field applied by the permanent magnet (1) to the core metal (21) becomes different in a portion from the mid-portion toward the one end and in a portion from the mid-portion toward the other end of the core metal (21). When the direction of the magnetic field applied to the core metal (21) is different in the portion from the mid-portion toward the one end and in a portion from the mid-portion toward the other end of the core metal (21), the magnetized direction changes partially in the core metal (21), and the output level of a pulse signal output from the coil wire (22) becomes low. Furthermore, because application of the magnetic fields in different directions makes emergence of the large Barkhausen effect indeterminate, a variation occurs in the output level of the pulse signal. It is difficult for a detection circuit subsequently positioned to accurately detect a pulse signal varying at such a low level. As a result, a change in the magnetized direction in the core metal (21) cannot be detected accurately.
When such a change that is difficult to predict in the magnetized direction occurs, it becomes difficult to accurately detect the rotational conditions of the rotational plate (200). Therefore, it is difficult to detect the rotational conditions of the shaft of a motor unit provided with such a rotation detector and detects rotations of the shaft using the rotation detector.